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Newcomers DBPro Corner / How do you make gravity?

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WoW is WOW
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Posted: 20th Oct 2003 11:18
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In a game, all things(unless it's a space game) are affected by gravity, so how do you simulate correct gravity in your game.
thanx.
RPG_Freak.
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adr
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Posted: 20th Oct 2003 12:21
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Every object I want to be affected by gravity has a velocity type in it. You needn't do it like this, but I actually can't imagine doing it any other way ...

define your types ...



and affect the velocity in your main loop...



Basically, the idea here is to create an increasingly negative value, and change your player's height by that amount. Because you're decrementing your y velocity by a constant every time, you get a very simple, but effective downward vertical acceleration effect...

ta da.

Can I ask you a Question?
What is it?
It's an interrogative form of sentence, used to test knowledge. But that's not important right now.
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Darkflame
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Posted: 20th Oct 2003 15:38 Edited at: 20th Oct 2003 15:40
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Yes thats the best way.

If you dont need accurate gravity you could just subtract a fix amount from a co-ordinate at set times. At no point needing a speed per object..mearly set up a loop to subtract from all objects nesscery.

This would be good enough, for say, Tetris, or other games where you need things to fall but not to accerate when falling.

Then again, true acceleration might add something to these games too
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adr~ Great sig, Leslie Nielsen rules.

"The man who says it cannot be done is generally interrupted by someone doing it." ~ Elbert Hubbard
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BatVink
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Posted: 20th Oct 2003 16:59
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"We need to get this man to a hospital"
"Why, what is it"
"It's a large building with patients and doctors, but that's not important right now"



"...And don't call me Shirley".

But to the point...do a search for "Physics", there's a couple of threads on here somewhere

BatVink (formerly StevieVee)
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GameKit
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Posted: 20th Oct 2003 17:40
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If i remember right, perfect gravity would be fall speed * 1.9 every second... so if your game were running at sixty frames per second it would be falls#=falls#*(1.9/60)... so i guess the best way to do it would be... falls#=falls#*(1.9/screen fps())...well...this is how it works in reality, but some games you would want a slower fall rate... in that case just increase 1.9 into 1.95 ect...

try inserting something like this into your main loop...


well... i hope that helps...

If It Wern't For Bad Apples, Good Apples Would Only Be Average...
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Hamish McHaggis
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Posted: 20th Oct 2003 19:18
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There is a very simple example on my site (click the site button below).

Brains are for idiots.

Athelon XP 1400 Plus - Nvidia Geforce MX400 - 256mb RAM
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zircher
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Posted: 21st Oct 2003 03:32
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If one G is 32 feet per second per second, why 1.9? Should it not be 9.8 as in meters? [Assuming 1 unit is equal to 1 meter...]
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TAZ
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GameKit
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Posted: 21st Oct 2003 17:25
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hmmm... taz, i think your right... its 9.8 instead of 1.9...my bad...

1.9/60=0.031... my code wouldn't work, the player wouldn't fall
9.8 should be correct...

If It Wern't For Bad Apples, Good Apples Would Only Be Average...
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Hamish McHaggis
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Posted: 21st Oct 2003 18:39
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But often in games, 1 unit doesn't equal 1 metre .

Brains are for idiots.

Athelon XP 1400 Plus - Nvidia Geforce MX400 - 256mb RAM
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Algae Man
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Posted: 22nd Oct 2003 01:58
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Yes, using '9.8' as the acceleration is really pointless. It could be any arbitrary amount, depending on the scale of your objects.
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zircher
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Posted: 22nd Oct 2003 06:34
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That's why I put that 'asuming' clause in there. However, you would need to multiply 9.8 by whatever scale that you happen to use. For example, 1.9 would be about right if one unit was equal to 5 meters.
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TAZ
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Darkflame
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Posted: 22nd Oct 2003 14:31
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9.81 is essiental because to have accurate gravity it is that much per-second-per-second.

An arbitary amount simply wont do if your dealing with any degree of temporal precision grater then 1 second.

v(t) = v(0)-9.81*(t^2)

v(t) = Velocity at time t.
v(0) = velocity at time 0.
-9.81 = rough value for G. (ie the acceleration).

"The man who says it cannot be done is generally interrupted by someone doing it." ~ Elbert Hubbard
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